In general, water-absorbent, hydrogel-forming polymeric material (such as material known as absorbent gelling material, AGM, for example formed from polyacrylate polymers), useful in absorbent articles (such as diapers), need to have adequately high sorption capacity in order to perform satisfactory. Furthermore, it may be preferable to also have adequately high gel strength. The sorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Together with other properties of the gel, gel strength relates to the tendency of the swollen polymer particles to resist deformation under an applied stress. The gel strength needs to be high enough in the absorbent member or article, so that the particles do not deform and fill the capillary void spaces to an unacceptable degree, causing thereby so-called gel blocking. This gel-blocking inhibits the rate of fluid uptake or the fluid distribution, i.e. once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent article and leakage from the absorbent article can take place well before the water-absorbent polymer particles are fully saturated or before the fluid can diffuse or wick past the “blocking” particles into the rest of the absorbent article.
When developing new water-absorbent polymeric materials or absorbent articles therewith, manufacturers of such materials or articles measure thereto frequently measure the capacity and gel strength of such newly developed materials.
The gel strength of such particulate materials is for example determined by swelling these particles in saline solution and determining the Saline Flow Conductivity test method, well know in the art, and for example described in U.S. Pat. No. 5,599,335, U.S. Pat. No. 5,562,646 and U.S. Pat. No. 5,669,894.
The inventors found however that during development of new water-absorbent hydrogel-forming polymeric materials it would be useful to be able to measure the gel strength of such hydrogels. The inventors also found that it is desirable to be able to test the gel strength of the material directly after production, of for example the base polymer, and for example before further surface cross-linking and/or before drying or particle formation steps. The inventors found that this can help in determining the impact of specific chemistry of the base polymer or specific properties of the base polymer on the intrinsic gel strength, without the influence of drying steps, particle formation steps etc. Furthermore, there is a need to determine the intrinsic gel strength with a method that reflects better the in-use situation, e.g. when a wearer wears an absorbent article (such as an infant wearing a loaded diaper (full or urine) and moving with the diaper, sitting on the diaper etc.). Furthermore, the inventors found that there is a need for a test method that can measure the intrinsic gel strength on hydrogels of the water-absorbent polymeric material, comprising high levels if fluid (e.g. reflecting a real-use situation of a (fully) loaded diaper).